A double integral type A/D converter integrates an input voltage over a certain period of time, then integrates a reference voltage on behalf of the input voltage, measures the time required for an output voltage of an integrator to return to an initial voltage, and thus provides an analog-to-digital (A/D)-converted value. An A/D converter described in Patent Document 1 (JP-A-10-4353) converts a voltage to be measured into a digital value on the basis of the ratio of an integration time required for integration of a voltage corresponding to the voltage to be measured to an integration time required for integration of a predefined known voltage. Herein, the voltage corresponding to the voltage to be measured and the predefined known voltage are integrated so that a change in an output of the integrator resulting from the integration of the voltage corresponding to the voltage to be measured will be equal to that resulting from the integration of the predefined known voltage. A means for measuring the integration time includes a delay circuit in which a plurality of concatenated inverter circuits sequentially inverts pulses constituting a pulsating signal and propagates the pulsating signal. The integration time is encoded by regarding as a time resolution a time equivalent to a phase difference between adjoining ones of the pulses in the pulsating signal that are sequentially sent from the plurality of predetermined inverter circuits.
The circuitry of an analog unit included in a double integral type A/D converter is relatively simple. For a recent micromachining process of fabricating semiconductors, it is cost-effective that the analog unit be minimized. Moreover, a problem of a long conversion time underlies related arts. The employment of the A/D converter described in the Patent Document 1 makes it possible to shorten the conversion time, though the measuring means must be modified.
Normally, an A/D converter analog-to-digital (A/D)-converts a signal which a multiplexer selects from among a plurality of analog signals. A conventional double integral type A/D converter starts integrating a voltage at an initial voltage of, for example, 0 V. When conversion is completed normally, an integrated voltage is returned to the initial voltage. However, when an interrupt for A/D conversion of other analog signal assigned high priority occurs during conversion of a certain input signal, since the integrated voltage is different from the initial voltage, initialization must be performed in order to initialize the charge in an integrating capacitor. The initialization requires a considerable time. Consequently, the conversion time cannot be further shortened. On the contrary, when the initialization time is shortened, precision deteriorates.